1. Field of the Invention
The present invention is directed to technology and designs of efficient ultrasound bulk wave transducers for wide frequency band operation, and also transducers with multiple electric ports for efficient operation in multiple frequency bands, for example frequency bands with a harmonic relation, where it is possible to receive the 1st, and/or 2nd, and/or 3rd, and/or 4th harmonic frequency bands of the transmitted frequency band.
2. Description of the Related Art
In medical ultrasound imaging, one uses a variety of center frequencies of the transmitted pulse to optimize image resolution for required image depth. To image deep organs one can use frequencies down to xcx9c2 MHz, while for shallow depths one can use frequencies higher than 10 MHz.
In many cases one also transmits an ultrasound pulse in one band of frequencies, and receive the back scattered signal in a second band of frequencies. This is for example done in 2nd harmonic imaging of tissue, where the receive band is centered around the 2nd harmonic frequency of the transmit pulse band. Nonlinear elasticity in the tissue distorts the forward propagating pulse, which increases the higher harmonic content in the pulse with depth. This method considerably reduces noise in the ultrasound image.
Second harmonic imaging is also used for the detection of ultrasound contrast agent. As the nonlinear elasticity of the contrast agent is very strong, it is also interesting to use a receive band centered around higher than the 2nd harmonic band, for example the 3rd or 4th harmonic component of the transmit frequency band.
It is also useful to transmit an ultrasound burst with two separate frequency bands, both for imaging of soft tissue and ultrasound contrast agents. The non-linear effects will then introduce new frequency bands in the scattered signal, centered around sums and differences of the transmitted center frequencies. When the center frequencies of the transmitted frequency bands coincide, the difference frequency is referred to as a sub harmonic frequency component produced by the non-linearity of the tissue or contrast agent elasticity.
Traditional ultrasound transducers for medical imaging have limitations for such applications in that they are efficient over a limited band of frequencies. The active material in the transducers, is usually a plate of piezoelectric ceramic that vibrates in thickness mode. Other piezoelectric materials like the crystal LiNbO3, or the polymer PVDF, are also sometimes used. In the following we mainly refer to ceramic materials while it is understood that other piezoelectric materials can be used in the same manner.
The ceramic has much higher characteristic mechanical impedance (Zxxcx9c34MRayl) than the tissue (Zxxcx9c1.5MRayl), and the energy coupling between the tissue and the ceramic plate is therefore by nature very low. To improve this energy coupling, the plate is operated around xcex/2 resonance when the backing mount has a lower characteristic impedance than the piezoelectric plate, or xcex/4 resonance when the backing mount has a higher characteristic impedance than the piezoelectric plate. The resonance increases the amplitude of the thickness vibrations, hence improving the tissue/ceramic energy coupling at the resonance frequency. The resonance, however, gives a limited bandwidth of the energy coupling, limiting the minimal pulse length transmitted through the transducer.
To increase the bandwidth of the energy coupling, impedance matching layers are commonly used between the tissue and the ceramic plate to raise the mechanical impedance seen from the plate towards the tissue. Further improvement in the bandwidth of the tissue/ceramic energy coupling, is obtained with the well known ceramic/polymer composite materials. Such materials are made by dicing grooves in the ceramic plate and filling the grooves with softer polymer, a process that produces a composite ceramic/polymer material with mechanical impedance Zxxcx9c12-20MRayl, substantially lower than for the whole ceramic.
Even with these techniques, it is difficult to produce efficient energy coupling bandwidths larger than xcx9c80% of the center resonance frequency, limiting the bandwidth to xcx9c35% for 2nd harmonic imaging, and making it impossible to use higher than the 2nd harmonic component of the back scattered signal for imaging. The reason for this is that the transducer plate is the dominant resonant layer in the structure, and the electrodes are placed on the surface of the piezoelectric layer so that the electrode distance becomes large at high frequencies.
For improved bandwidth with 2nd harmonic imaging, there has been presented transducer structures with two piezoelectric layers with electrodes on the surfaces that gives a dual band performance. Through coupling of the electrodes one is able to transmit selectively in a low and a high frequency band, and receive selectively in the same low and high frequency bands. However, the presented patents make less than optimal use of the multilayer design for widest possible bandwidth, and the flexibility for selecting transduction in different frequency bands is limited.
The present invention presents a new layered transducer structure including optimized examples of the design that provides wider transduction bandwidths than previous designs, allowing transmission and reception of ultrasound pulses over two octaves, i.e. from a 1st to a 4th harmonic component of the lowest frequency band. The invention also provides details of efficient manufacturing of the layered structure. The method to increase the bandwidth is also useful for single piezoelectric layer transducers, increasing the relative bandwidth of such transducers to above 100%. This makes single piezoelectric layer transducer efficient for 2nd harmonic imaging and also for 1st harmonic imaging in different frequency bands.
The invention further presents methods for electronic selection of a wide variety of combinations of electro-acoustic ports in multi-layered transducers, for electronic selection of the efficient transduction bands of the transducer. This allows the transmit ultrasound pulses with frequency components in multiple bands, say both a 1st and a 2nd harmonic band, with transmitter amplifiers that switches the drive voltage between +V, xe2x88x92V, and zero. The invention further devices methods of combining the received signals from multiple electric ports for parallel reception of signals over two octaves of frequencies, or in a 1st, 2nd, 3rd, and even 4th harmonic component of the transmitted frequency band.
The invention presents solutions to the general need for ultrasound transducers that can efficiently operate over a large frequency band, or in separated frequency bands both for transmit and receive, so that: 1) one can use the same transducer to operate with several ultrasound frequencies to select the optimal frequency for the actual depth, 2) one can obtain wider transmit and receive bands with 2nd harmonic measurements and imaging, 3) one can receive higher than the 2nd harmonic component of the transmitted pulse, for measurement and imaging of objects with high non-linear elastic properties, and 4) one can transmit a complex ultrasound burst containing frequencies in more than one frequency band, and receive signals in frequency bands centered around sums and differences of the transmitted center frequencies.
According to the present invention, such wide band or multi band operation of the transducer is achieved through three design attributes:
1. Overall structure: The total transducer is composed of a set of piezoelectric and purely elastic layers, mounted on a backing material with so high absorption that reflected waves in the backing material can be neglected. The layers are grouped into: 1) a core, high impedance section that contains the piezoelectric layers, 2) a load matching section of elastic impedance matching layers between the high impedance section and the load, and 3) possibly also a back matching section of elastic impedance matching layers between the high impedance section and the backing material.
The high impedance section is composed of piezoelectric and possibly also purely elastic layers, where all layers of this section have close to the same characteristic impedance Zx, which is the highest value in the whole structure. As the exact value of the characteristic impedance is difficult to control and can vary even within a piezoelectric layer, the requirement of constant characteristic impedance within this section must be viewed as fuzzy and imprecise where up to a 20% variation can be tolerated, as discussed below. The basic requirement is that the high impedance section functions as a unity when determining resonances of the structure. The resonances of the structure is then determined by the total thickness Lx of the whole high impedance section, and not by the thickness of the individual piezoelectric layers.
The highest sensitivity of the transducer is obtained by minimizing the power transmitted into the backing. This is obtained by either selecting the lowest or highest possible characteristic impedance of the backing material so that the velocity reflection coefficient at the backing interface is close to +1 or xe2x88x921. Matching layers between the piezoelectric section and backing can be used to reduce the power transmitted into the backing in certain frequency ranges, for example to increase the sensitivity for high frequencies in a band. A problem with such matching is that its resonant nature can reduce the overall operating band of the transducer.
The load matching layers are according to well known methods selected to transform the load characteristic impedance to a higher value close to Zx, over as large frequency range as possible. This is done with standard methods where one for example can choose equal ripple, or an exponential tapering, of the reflection coefficient between the high impedance section and the load matching section, with xcex/4 layer thickness of the matching layers at the center of the efficient matching band. With such an arrangement of the layers, the reflection coefficient between the high impedance section and the load matching section can be made small over the effective frequency range of the impedance matching. The invention also devices a new method of manufacturing such layers as metal/polymer composites similar to the high impedance elastic layers described below.
When the impedance seen from the piezoelectric section towards the load deviates from Zx, one gets resonances when the sum of the roundtrip propagation phase (2 kLx) through the high impedance section and the phases of the reflection coefficients at the load and back interfaces of the high impedance section, is a whole number of 2xcfx80. Here k=xcfx89/c is the wave number at the angular frequency xcfx89 in the piezoelectric section with wave propagation velocity c.
With proper placement of electrodes as discussed under point 2 below, the resonance gives improved phase of the electric impedance of the electric port, hence giving improved sensitivity of the transducer in the resonant bands. According to the invention, thickness resonances in the high impedance section is used to boost the transduction efficiency at the lower and upper frequencies where the load matching section starts to become inefficient, hence increasing the active transduction band of the transducer. To achieve this effect, the thickness of the high impedance section is increased by added elastic layers, introducing resonances of this section on the low and high side of the efficient band of the load matching.
The added elastic layers in the high impedance section can be loaded or unloaded piezoelectric layers, which already have the same characteristic impedance as the other piezoelectric layers of this section. The characteristic impedance of composite piezoelectric materials can also be brought down in the 12-20 MRayl range, where one can find other materials with similar characteristic impedances, like aluminum (Al: Z0xcx9c17.3MRayl) and magnesium (Mg: Z0xcx9c10MRayl) materials, and the semiconductor silicon (Si: Z0xcx9c19.5MRayl). Conducting metals and highly doped Si can also be used as electrodes in the structure, and transistor amplifiers and switches can also be integrated on Si-layers. Excitation of transversal modes and shear waves in the elastic layers can introduce problems, depending on the dimensions. In such cases, the invention devices a solution to attach layers of silver (Ag: Z0xcx9c38MRayl), zirconium (Zr: Z0xcx9c30.1MRayl), or zinc (Zn: Z0xcx9c39.6 MRayl) directly to the undiced, whole ferrolectric ceramic material. Other actual materials are alloys like brass (Z0xcx9c36MRayl) or cast iron (Z0xcx9c33MRayl). These materials have characteristic impedances that are sufficiently close to the ceramic materials, and can be diced together with the ceramic layers to form a final metal/ceramic/polymer composite. The elastic layers of the metal/polymer composites can then be used as part of the electrodes as they provide metallic connection directly to the ferroelectric ceramic slabs, as discussed below. The invention also devices similar methods for manufacture of high impedance load matching layers with reduced lateral coupling. Mixtures of polymer with tungsten or other heavy powders can also be used for elastic layers in the high impedance section, albeit they have larger power absorption and hence reduces sensitivity compared to the other solutions.
2. Electrode placement. Conducting electrode layers are inserted at the surface of the piezoelectric layers in the high impedance section, to divide the high impedance section into elastic and piezoelectric layers separated by the electrodes. Two such electrode layers with an intermediate piezoelectric layer, constitute an electric layer port. The placement of the electrodes are selected so that for the active frequency bands of the port, a high thickness vibration amplitude of the piezoelectric layers between the electrodes is found.
For widest possible bandwidth, the back electrode is located at the interface between the backing mount and the high impedance section (no matching layers to the backing), as this location for all frequencies is either an antinode (for low impedance backing) or a node (for high impedance backing). The other electrode is then at the center of the actual frequency band selected at the antinode in front of the backing interface. This gives maximal thickness vibrations of the material between the electrodes at the center frequency, and as the back electrode is stationary relative to the standing wave pattern, we get a widest possible bandwidth of the electric pick-up.
Maximal electric pick-up is also obtained when there is an uneven number of half wave lengths between the electrodes when the back electrode is at an antinode (low backing impedance), or an uneven number of quarter wavelengths when the back electrode is at a node (high back impedance). In some situations one wants to use a limited transduction bandwidth of the transducer to filter the ultrasound pulse, for example to attenuate 2nd and 3rd harmonic components in the transmitted pulse with harmonic imaging. This can be furthered by positioning the back and front electrodes so in the standing wave pattern, that they vibrate with the same phase and amplitude at these frequencies.
3. Combining electric ports. The high impedance piezoelectric section can contain several piezoelectric layers covered with electrodes to form one electric port per layer. The signals for several electric layer ports are then favorably combined to influence the overall transfer function. The simplest examples are that the electrodes are galvanically connected to form a series or parallel coupling of two or more electric layer ports into a new electric resultant port. Coupling the electrodes of the layers together so that the voltages across the layers are the same (with voltage polarity defined relative to the polarization direction of the piezoelectric material), and the current into the resultant port is the sum of the currents in the layer ports, one obtains electrical parallel coupling of the layers. Coupling the electrodes of the layers together so that the voltage across the resultant electric port is the sum of the voltages across the layer ports, while the currents in the layer ports are the same as the current in the resultant port, gives an electric series coupling of the layers. In this galvanic coupling of the ports, it might be necessary to isolate electrodes between neighboring layers, or use opposite direction of the polarization of neighboring layers according to well known principles. One can also at transmit steer the voltages on individual electrodes so that one selectively obtain electrical parallel or series coupling of electric layer ports, as described in FIG. 12. Electrical anti-serial and anti-parallel coupling of the ports, where the currents or the voltages, respectively, of the ports have opposite polarity, are also actual to obtain specific transfer function as described in the specification below.
With galvanic coupling of the electrodes, the current in one set of layers influences the current in other layers so that one gets electrical coupling of the vibrations of all participating layers in the resultant port. Other types of combinations of the layer ports or resultant ports in receive mode, can be obtained by combining the signals after preamplifiers from the layer ports, possibly after filtering of the signals, into composite signal ports as described in FIG. 12. In this case the vibrations of the participating layers are unmodified by the combination.
One hence typically can have situations where layer ports are galvanically combined to produce resultant multi-layer ports, for example by parallel coupling of layer ports to obtain reduced electric impedance of the galvanic resultant ports. These galvanic resultant ports can again be combined electronically to form new composite ports that are electronically selectable.
The invention hence describes a general transducer concept that can be adapted for efficient operation of a single transducer in such a wide band of frequencies that multi frequency band operation can be achieved with the same transducer. The patent also applies to the design of individual elements of an ultrasonic transducer array. The description below shows specific designs based on the general principle introduced, that is particularly useful for sub, second, third, and fourth harmonic measurements and imaging, and combinations thereof.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.